Energy and Human Ambitions on a Finite Planet, 2021a

9 Climate Change 149
simulation allows us to evaluate the best-case (<strong>and</strong> unrealistic) scenario
of instant, complete replacement, to put a limit on how much benefit
such a move brings. Figure 9.12 shows what happens. The rate of CO 2
emission would immediately drop to 1.8 ppm v /year. 37 That definitely
helps, but the total emission by 2100—if carrying on at today’s energy
dem<strong>and</strong> via fossil fuels—would climb to 268 ppm v . The effect would
more than double the 123 ppm v that we’ve already contributed to the
atmosphere, <strong>and</strong> would approximately double the pre-industrial CO 2
level in the atmosphere, leading to a forcing of 3.6 W/m 2 <strong>and</strong> ΔT ≈ 2.9 ◦ C
(summarized in Table 9.5). So as beneficial as the termination of coal
would be, any path that involves carrying our fossil fuel use forward at
today’s levels—even substituting the best form for the worst form—does
not look promising.
37: This would be about 70% the present
rate of 2.6 ppm v per year.
See if you can replicate these
numbers!
120
1.50
2100: 1.8 ppm V /yr
110
Total: 268 ppm V
CO2 ppmV annual contribution
1.25
1.00
0.75
0.50
0.25
oil
gas
coal
cumulative CO2 ppmV contribution
0.00
1800 1850 1900 1950 2000 2050 2100
year
100
90
80
70
60
50
40
30
20
10
coal
oil
gas
0
1800 1850 1900 1950 2000 2050 2100
year
Figure 9.12: CO 2 rise if instantly substituting
coal (worst CO 2 intensity) with natural
gas (best CO 2 intensity) <strong>and</strong> then maintaining
current levels for the rest of this century.
Our annual contribution would drop from
2.6 ppm v /yr to 1.8 ppm v /yr based on this
substitution, <strong>and</strong> the total accumulation
would be 268 ppm v by century’s end (2.2
times the accumulation to date). The associated
temperature rise would be 2.9 ◦ C.
The emphasis, then, should be to taper off fossil fuel use so that we wean
ourselves of dependency. The transition could be fast or slow. A slower
version might target the year 2100 for a full termination of fossil fuels.
Figure 9.13 shows an idealization of what this might look like. Notice
that the resulting curves are roughly symmetric, in that the downslope
is not terribly different from the upslope. Let’s pause to reflect on how
incredible <strong>and</strong> fast the rise of fossil fuels has been. A descent as steep
as the rise represents change at an astounding pace—which would be
pretty disruptive in the best circumstances. In the absence of suitable
substitutions, this would be a tremendously difficult journey, but one
we may be forced to make by any number of paths. 38 In any case, the
eventual added CO 2 would end up at 235 ppm v —almost doubling what
we have already emitted, <strong>and</strong> nearly doubling the pre-industrial CO 2
level in the atmosphere. The forcing in this case would be 3.3 W/m 2 <strong>and</strong>
ΔT ≈ 2.6 ◦ C.
Reducing fossil fuel use even more quickly, tapering to zero by 2050,
results in Figure 9.14. The descent is alarmingly steep, <strong>and</strong> difficult to
imagine happening in practice unless major disruptions 39 force this
upon us. In any case, should we manage such a feat, our total CO 2
38: . . . driven by policies, markets or—more
certainly—resource limits
Again, follow along for good
practice.
39: . . . resource wars, devastating climate
change repercussions
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.